We encountered cold solder joints on all of the 0603 components on the bottom side of a small batch of board assemblies, yet all the other larger components had good solder joints. These components are all near via holes, but this does not seem to be related to the cause. We replaced the solder paste on a new batch of assemblies, and the problem was eliminated.

The solder paste used for the problem batch of assemblies had exceeded the 12 hour setup usage time we apply to solder pastes in our facilities by 1 hour. Was the problem likely due to the solder paste exceeding our 12 hour usage limit, or something else?

L. S.

Experts Comments

Assuming that the reflow profile was long and hot enough to melt the solder, what you are seeing is most probably "graping" or improper coalescence, which is mostly seen on smaller components.

The reason for graping is due to a poor oxidation barrier in the paste. Oxidation barrier is the ability of the flux (in the paste) to protect the solder particles from oxidizing during reflow. When solder particles are not protected from oxidation, even though melting happens, complete coalescence does not happen and clumpy looking joints similar to cold solder are observed (figure below).

Graping is more prevalent in smaller components as the paste volume deposited is lesser. As paste volume decreases, there is lesser flux volume per unit surface area. The graph below shows that as the stencil aperture becomes smaller, the flux volume (dark green curve) is lesser than the solder surface area (light green curve).

So it is very important to use a solder paste (a) with an excellent oxidation barrier; (b) that maximizes paste transfer volume (to increase flux volume) especially for the smaller deposits; (c) that has a long stencil life during paste usage without flux degradation.

What you are seeing could be due to the poor oxidation barrier of the paste in general- OR inconsistent / lesser paste deposits for the 0603s and/or flux volatilization; due to exceeding the paste usage time

Karthik VijayTechnical Manager - EuropeIndium Corp.Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.

In some cases, small solder deposits that look like cold solder could actually be graping, which looks very similar. Graping is a clumpy appearance on the outside of small solder deposits that forms because oxidized particles on the outside of the solder joint do not coalesce properly.

Another clue is that larger components had good soldering, while smaller components didn't. Because of this, I would guess that what you experienced was actually this graping phenomenon, which could have been exacerbated by the exceeded usage limit as well.

Leaving the paste out longer than usual could have contributed to excess oxide forming on the surface particles and inhibiting coalescence. In the future, to avoid this problem, limit the heat exposure for your assembly to minimize oxide formation and graping.

Brook SandyProduct Support Specialist Indium CorporationBrook is Product Support Specialist for PCB Assembly Materials at Indium Corp. With a background in Chemical Engineering and previous experience with conductive adhesive new product development, she provides expertise in choosing the right materials and processes to optimize PCB assembly operations.

Yes, We call it as Solder Paste Management, its an very important factor for Solder joints.

I would of thought it could of been more like you have sheared thinned the paste too much especially if you over pressurise the squeegee with Lead Free in mind. Pressures of a maximum 1Kg per 50mm blade length for LEad Free should be used, nothing higher.

If you over pressurise then you will have flux rich areas as well as flux starved which would definatley be more pronounced on smaller devices. Hence moving to fresher paste properly mixed has made a difference. Worth checking out and very simple to fix.

I can't honestly believe by exceeding the meager 12 hour working life by one hour has made that much of a problem for you BUT it will have extended the over pressurising of the paste and subsequent the poor mix of flux contained within the flux

Worth checking the print settings to confirm.

Greg YorkTechnical Sales ManagerBLT Circuit Services LtdGreg York has twenty two years of service in Electronics industry. York has installed over 350 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Since changing the used solder paste with fresh one
improved the solder wetting results you can attribute the problems to the
solder paste condition. From my perspective the 12 hours exposure time for the
solder paste is excessive regardless of the conditions inside the printing
machine (temperature and humidity control) or the solder paste type and
manufacturer. Excessive exposure will create a good environment for oxidation.
Since part of the solvents from the paste will evaporate, the smaller pads are
going to be affected when it comes to proper reflow (the flux cannot be
properly activated anymore).

Georgian SimionEngineering and Operations ManagementIndependent ConsultantGeorgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.Contact me at georgiansimion@yahoo.com.

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